Tutorial by Examples: backreference

Since Groups are "numbered" some engines also support matching what a group has previously matched again. Assuming you wanted to match something where two equals strings of length three are divided by a $ you'd use: (.{3})\$\1 This would match any of the following strings: "abc$...

Using escaped brackets, you can define a capturing group in a pattern that can be backreferenced in the substitution string with \1: $ echo Hello world! | sed 's/\(Hello\) world!/\1 sed/' Hello sed With multiple groups: $ echo one two three | sed 's/\(one\) \(two\) \(three\)/\3 \2 \1/' three ...

In PCRE, matched groups used for backreferences before a recursion are kept in the recursion. But after the recursion they all reset to what they were before entering it. In other words, matched groups in the recursion are all forgotten. For example: (?J)(?(DEFINE)(\g{a}(?<a>b)\g{a}))(?<a...

Problem: You need to match text of a certain format, for example: 1-a-0 6/p/0 4 g 0 That's a digit, a separator (one of -, /, or a space), a letter, the same separator, and a zero. Naïve solution: Adapting the regex from the Basics example, you come up with this regex: [0-9]([-/ ])[a-z]\...